Addition-crosslinking silicone compositions crosslink by reacting aliphatically unsaturated groups with Si-bonded hydrogen (hydrosilylation) in the presence of a catalyst, typically a platinum compound. Because the crosslinking reaction begins at the point when all the essential constituents are simultaneously present, addition-crosslinking silicone compositions have hitherto been prepared almost exclusively in the form of two-part (two component) formulations, where the makeup of the individual components is designed such that all of the three essential constituents are not simultaneously present until the components have been mixed. Usually, one of the components comprises the alkenyl-functional polyorganosiloxane and the platinum catalyst, and the other component comprises the SiH-functional crosslinking agent, if desired in combination with alkenyl-functional polyorganosiloxane. After the individual components have been mixed, complete cure may be effected at room temperature to give a silicone elastomer, although curing usually takes place at an elevated temperature.
The two-part system for addition-crosslinkable silicone compositions is associated with numerous disadvantages, such as logistics, the high risk of contamination by traces of platinum and the necessity for an additional mixing step. Although a ready-to-use composition is obtained once the components have been mixed, this composition has a severely restricted pot life, even at room temperature. This short pot life requires, first, that the composition be used very quickly, and second, frequent cleaning of the storage containers, metering systems, processing machinery, etc. is performed, since any material remaining, for example as a result of back-mixing or adhesion to walls, will ultimately gel.
These disadvantages have encouraged many attempts to provide addition-crosslinking silicone compositions in the form of one-part formulations (IP systems). Since in a 1P system all of the constituents needed for the crosslinking are present together, the problem is fundamentally that of finding some way of suppressing the premature onset of the crosslinking reaction, which normally proceeds significantly even at room temperature. Known methods for controlled adjustment (extension) of the pot life of addition-crosslinking compositions are, for example, the use of inhibitors, which are able to considerably reduce the activity of the platinum catalyst at room temperature. Examples include phosphorus compounds in combination with peroxides as disclosed in U.S. Pat. No. 4,329,275 or azodicarbonyl compounds as disclosed in EP-A-490 523. Varying the type and content of these inhibitors can extend the pot life as desired, but increasing pot life by inhibitor use is also inseparably associated with a disadvantageous effect on crosslinking performance. This applies in particular if the pot life is extended to several months using high inhibitor contents: increased initiation temperatures, low crosslinking rates, and even under-crosslinking are the result.
Another fundamentally different method for extending storage life of 1P systems is to encapsulate the platinum catalyst in a finely divided material which does not release the platinum until the temperature has risen. This can be done, for example, by microencapsulating the platinum catalyst using a thermoplastic silicone resin or an organic thermoplastic, as described, for example, in EP-A-363 006, but this is a relatively complicated procedure. A third method is to select the catalyst from specific platinum complexes whose activity is designed to provide sufficiently rapid hydrosilylation at elevated temperatures, but slow reaction at room temperature such that pot lives of a number of months are achieved. Addition-crosslinking compositions of this type comprising platinum complexes have been described, for example, in EP-A-583 159 and DE-A-36 35 236. Although the compositions described have markedly improved pot lives with, in some cases, sufficiently high crosslinking rates, there remains a need to improve the pot life and crosslinking rate of addition-crosslinking compositions having one-part formulations by using higher performance platinum catalysts without having to accept the disadvantages described above. This object is achieved by the present invention.